SEMICONDUCTOR OPTICAL MODULATOR

Disclosed is a semiconductor optical modulator which has: semiconductor input-light waveguide with an incidence end; a semiconductor Y-branch element for branching light from the semiconductor input-light waveguide into two equal lights; first and second semiconductor optical waveguides for propagating the two lights, respectively branched by the semiconductor Y-branch element; first and second electrodes provided on the first and second optical waveguides, respectively; a semiconductor optical coupling element for coupling the two lights passed through the first and second optical waveguides; and a semiconductor output-light waveguide for leading light coupled in the semiconductor optical coupling element to an emission end of the semiconductor output-light waveguide; wherein the semiconductor optical modulator is provided with a means for producing a phase difference of 180.degree. between the two lights input to the semiconductor optical coupling element when equal voltages are applied to the first and second electrodes.